1. Field of the Invention
This invention relates to a biaxially oriented, laminated polyester film and, more specifically, to a biaxially oriented, laminated polyester film useful for magnetic recording media, stencil films and package materials. In particular, the invention is useful as a base film of a high-density magnetic recording medium for data storage such as an S-DLT (Super Digital Linear Tape) and an LTO (Linear Tape Open) having a capacity of 40 GB or more.
2. Description of the Prior Art
Recently, in the field of magnetic recording media, the technology for making a magnetic recording medium larger in capacity and higher in density has been developed, and accompanying therewith, it is required to make the base film of the medium flatter and thinner. Although the flatness of the base film is very important to achieve a good electromagnetic conversion property, if it is too flat, the roll formation of the film in the film manufacturing process, for example, in a process for winding the film as a roll, may deteriorate. Also in a process for processing a film, if the film is too flat, undesirable friction may occur between the film and a magnetic head and it may cause poor running ability of the film or may cause powder chipped from the film which induces dropout and, therefore, it is necessary to roughen the surface of the base film appropriately to ensure an appropriate slipperiness.
As a means for solving the above-described problems, a biaxially oriented, laminated polyester film having a laminated structure and having different slipperinesses on the surface and the back surface is known, wherein, in use for high-density magnetic recording media, for example, a surface to be provided with the magnetic layer is made flat and the other surface is made uneven to ensure a good running ability (for example, JP-A-2-77431).
On the other hand, it is also known that when an undesirable irregularity in thickness is present in a base film, it may cause irregularity in film properties ascribed to nonuniform heat transfer and may cause deterioration of the roll formation of a film roll. Especially in the use for magnetic recording media, because such an irregularity in thickness causes an irregularity in application in a film processing step, it should be improved. Films improved with irregularity in thickness in the longitudinal and transverse directions also are well known as a means for solving such problems. For example, JP-A-11-115043 discloses a method for preparing a film excellent in uniformity of thickness by optimizing orientation and crystallization of the film controlled by heat treatment in the longitudinal stretching process in the film manufacturing process. Further, JP-A-4-96370 discloses a method for making the film thickness uniform in the transverse direction by specifying the design of a die.
However, in a tape for data storage on which an extremely thin and flat magnetic layer and back coating layer are applied as compared with those in the conventional tapes to satisfy the large capacity/high density requirements, even if such a film controlled with irregularity in thickness and controlled with flatness as described above is used, required PES (Positioning Error Signal, a value representing an amount of positional shift in the transverse direction and a value of standard deviation o) and a required electromagnetic conversion property could not be obtained.